There are a variety of types of digital projection systems that have been developed and gained widespread use in recent years. Digital projection systems generally include a spatial light modulator for producing an image signal, and a light source to illuminate the light modulator and associated optical elements to project a desired image to a projection surface. Types of spatial light modulators currently in use for projection systems include liquid crystal display (LCD) systems (both reflective and transmissive), liquid crystal on silicon (LCOS) displays, and digital mirror devices (DMD), among others.
Many digital projection systems require that the light source provide a rapid sequence of primary colored light (typically red, green, and blue) during each image frame. The light modulator, in synchronization with the timing of the switching of red, green and blue colors, sequentially produces images corresponding to the red image signal, green image signal, and blue image signal for a given image frame. The separate color images are integrated in the eye of the viewer to give the appearance of a single full color image.
Providing the sequence of primary colors can be done in various ways. Many projection systems include a single high output lamp (e.g. an arc lamp, incandescent lamp, etc.) that provides white light, the light from the lamp being temporally divided into the sequence of primary colors via a color wheel having a sequence of color filters. The color wheel mechanically rotates in synch with the refresh rate of the light modulator, so that the white light beam sequentially passes through each color filter once during each image frame to provide the separate primary colors.
Unfortunately, projection systems that employ mechanical color wheels present certain drawbacks. The color wheel itself and the motor that drives it tend to be bulky, causing the associated projectors to also be larger and heavier. The mechanically rotating color wheel and its motor also produce unwanted noise. Additionally, high output incandescent projector bulbs are expensive, have a limited useful life, produce relatively large amounts of heat, and consume a relatively large amount of electricity, in addition to that consumed by the color wheel motor.
To address some of these problems, projector light systems that utilize LEDs have been proposed. LEDs have some advantages over conventional projection bulbs. They use less electricity per unit light output, and produce less heat. They can also be less expensive and more durable than high output projector bulbs. Additionally, LEDs can be rapidly switched to provide light of a desired color precisely in synch with an image modulator.
Unfortunately, individual LEDs provide far less light than a typical high output incandescent projector bulb. Consequently, to provide the quantity of light need, an array of many LEDs is required. It is useful for projection for the light from the LEDs in the array to be combined and redirected into a single output stream. While this can be done in many ways, some methods present undesirable aspects.
One method for combining and redirecting light from multiple LED sources is with the use of a dichroic x-cube or pair of dichroic plates. This device receives light from three different locations, and redirects the light into a common output stream, allowing red, green, and blue light streams to be combined into an effective white light source. Unfortunately, this method is bulky, and not well suited to working from a lighting array of three coplanar color sources. Additionally, expensive thin-film dichroic coatings may be required.
Another method for combining and aligning multiple color sources is with a stack of switchable diffraction gratings. Each grating in the stack is configured to maximize the diffraction efficiency of a different color of light. When the grating corresponding to a given light color is switched on, that light passes through the grating and is diffracted into the desired illumination direction. This configuration requires very precise alignment of the light sources and the stacked diffraction grating.